A digital communication system using ASK transmits $1$s and $2$s by changing the amplitude of a carrier signal between two values, A$1$ and A$2.$ If the bandwidth of the signal is $2$ kHz$,$ what is the maximum spectral efficiency $($in bps$/$Hz$)$ of the system? A$.1$ bps$/$Hz B$.2$ bps$/$Hz C$.3$ bps$/$Hz D$.4$ bps$/$Hz